This invention relates generally to automatic rearview mirror systems for vehicles and, more particularly, to rearview mirror systems utilizing electrochromic mirrors. The invention is especially adapted for use with the exterior mirrors on a vehicle and is particularly suited for use on automobiles, minivans, light trucks, small vans, and sport utility vehicles.
Electrochromic mirror technology has facilitated the ability to control the reflectivity level of both the rearview mirror in the vehicle passenger compartment and the mirrors mounted on the exterior of the vehicle by dimming the mirrors to a partial reflectance level. The partial reflectance levels of the exterior mirrors are commonly controlled by the circuit that also establishes the partial reflectance level of the interior rearview mirror. The exterior mirrors are dimmed to partial reflectance levels that are either the same as, or in proportion to, the partial reflectance level of the interior mirror. Such conventional control of the exterior mirrors in tandem with the interior mirror is commonly referred to as slave control of the exterior mirrors.
One difficulty with such control of exterior mirrors is that a drive signal line must be run from the control, typically located in the housing of the interior mirror, to the exterior mirror. The drive signal line is typically routed to the exterior mirrors along with supply lines for providing vehicle battery voltage or ignition voltage to a mirror heater, provided in order to remove dew and ice from the exterior mirror, and to the motors used to mechanically position the mirror about vertical and horizontal axes. The electrochromic mirror element is generally operated in a voltage range of between about zero volts and about two volts while the voltages supplied to the other exterior mirror components range between approximately 12 and 16 volts DC. Such combination of disparate voltage levels increases the risk that the vehicle's battery voltage, ignition voltage, or a 12-volt supply may be inadvertently applied to the electrochromic mirror element, especially during repair or replacement of the exterior mirror, thus damaging or even destroying the electrochromic element. Another disadvantage of such conventional system is that special wiring must be placed within vehicles which have automatic exterior rearview mirrors. This increases the number of vehicle configurations, as well as parts inventories, and all but prohibits the addition of a stand-alone automatic rearview mirror system to the exterior of the vehicle as a field retrofit in the after-sale market.
Furthermore, such slaved control of the exterior mirror by the circuitry of the interior automatic electrochromic mirror may be economically and/or physically disadvantageous in some vehicles. For example, in vans, and their like, there may not exist an opportunity to mount an interior mirror with an unobstructed field of view rearward. Likewise, for some minivans, light trucks and for large Class 8 trucks, which are heavy trucks that are equipped on their driver-side and passenger-side with large exterior rearview mirrors, with a mirror reflector area typically greater than about 500 cm.sup.2, it may be desirable, indeed may be necessary, to use electrochromic rearview mirrors on the driver-side or passenger-side, or both, only with either a conventional lower cost conventional prismatic mirror used within the interior cabin, or no interior mirror at all being within the interior cabin.
Controls for an automatic rearview mirror traditionally utilize two sensors, one aimed generally rearwardly of the vehicle, in order to detect glare incident the rearview reflecting surface, and one aimed in a direction other than rearward of the vehicle, typically forwardly, in order to detect general lighting conditions. One such control is disclosed in commonly assigned U.S. Pat. No. 4,886,960 issued to Keith Molyneux et al. for a CONTROL CIRCUIT FOR AN AUTOMATIC REARVIEW MIRROR. While such controls are suitable for use in automatic rearview mirrors positioned within the passenger compartment of the vehicle, difficulties are created by attempting to position such controls proximate the mirror element exterior the vehicle. Of primary difficulty is the positioning of the sensor that is directed other than rearward of the vehicle. In order to position a sensor to be aimed in a direction other than rearward of the vehicle, it would be necessary to provide a hole, or opening, or at least a non-opaque, light-transmitting window, in the exterior mirror housing through which the non-rearward facing sensor would be exposed to light. One difficulty with providing such separate opening or window is that the mirror is adjustable physically with respect to the housing. Therefore, either the control must be mounted separately from the mirror and interconnected with wiring, or separate leads must be run from the sensor to a control mounted to the mirror. Either way, the labor cost and complexity of the mirror is greatly increased. It would be disadvantageous to position the non-rearward facing sensor on a control moveable with the mirror because alignment between the sensor and the opening, or window, would be extremely difficult. Another difficulty with providing a separate opening, or window, in the mirror housing is that such opening, or window, exterior of the vehicle would have a tendency to become blocked with debris. In contrast with the surface of the exterior mirror, which is cleaned when it is perceived by the driver that the mirror is obstructed by debris, a separate opening, or window, in the mirror housing would not likely be cleared of debris in between washings of the vehicle. Further, any hole or opening in the mirror housing could be a potential port of entry for moisture from rain, carwashes, road splash, and the like, with a consequent propensity for deterioration, electrical malfunction, and/or damage to components within the housing.
Perhaps for these reasons, although it is known to use mirror control circuitry such as described in the Molyneux et al. '960 patent that involves two sensors in conjunction with exterior vehicular electrochromic mirrors, to date such two-sensor exterior electrochromic automatic rearview mirrors have not enjoyed commercial success, particularly when used as exterior mirrors on automobiles, minivans, light trucks, small vans, and sport utility vehicles where the mirror reflector areas are relatively small (within the range between above about 100 cm.sup.2 to below about 500 cm.sup.2) and where the mirror housings are correspondingly cramped.